Document phase 4 exact-ensemble streaming - #238

Completes the durable Phase 4 record required by discussion #232.

The document gives the once-only four-boundary proof, exact filter and numerical invariants, tracker fallback, restored virtual update hook, exception-safety argument, CLI compatibility, C++ object-layout rebuild caveat, focused/full test evidence, and explicit roadmap deferrals.

Measured result for code PR #237 against executable parent 8bd1676:

• exact 80-by-80 ensemble wall time: 3.110 +/- 0.098 s -> 2.347 +/- 0.023 s (1.325x; 24.5% lower)
• mean peak RSS: 89,184 KiB -> 7,595 KiB (91.5% lower; about 11.7x)
• raw and canonical output hashes are identical
• both build/test gates and the benchmark evidence Action passed

Evidence is committed in the private benchmark repository at 30730f0d0932d31019d8461087f921c7e5497ea5. The document marks the selected architectural change accepted and leaves the remaining roadmap items explicitly deferred to separate proofs and measurements.